It is common practice in the manufacture of metal cans, such as those used as containers for foods and beverages, to apply a protective coating to the cans. The provision of such a coating is especially important in the case of non-alcoholic carbonated beverages, to avoid attack by the contents. Where the cans are of the closed end ("two piece" or integral bottom) type, the protective coating is usually applied as a wash or flow coat to the can interior, exterior, or both, as the can stands inverted on a so-called coating conveyor. For coating the can interior, a spray nozzle projects the coating material up through the open mesh or flat wire network of the conveyor. The large excess of coating material drains off the can walls, through the coating conveyor.
The commonly used coating materials are free-flowing, hardenable resin-containing liquids. They are usually water based; that is, they are at least water dispersible and are generally water soluble. Because of their flowability, low solids content (which may be in the range of about 5-25 weight %), and the fact that the can surface is flooded with an excess of the coating material, the coating material runs downwardly on the can wall toward the lower edge of the inverted can. The film which remains on most of the sidewall is of acceptably uniform thickness, but at the lower edge the coating tends to collect to excessive thickness. This "bead" will not necessarily level out or drain off of its own accord, and unless special efforts are made to remove it, the bead after hardening will be present as an undesirable defect.
The can coating is commonly hardened by heating, for example by passing the can through a continuous oven wherein it is baked at an air temperature of about 425.degree.-475.degree. F. A hardened bead of excessive thickness remaining on the can may crack off, thereby leaving the underlying metal unprotected; moreover, the thick bead material may accumulate on and interfere with the flanging or lid applying machinery.
Modern can making and coating machines operate at extremely high rates, for example, machines producing 800 cans per minute on a 6 foot wide belt are common. These high rates of production, with large total quantities of coatings involved, and the importance of consistently achieving full but not excessive coatings, have presented a need for an effective method of preventing formation of excessively thick beads. It has been the objective of this invention to satisfy that need.